A conventional head up display (HUD) device is disclosed in PTL 1. As shown in FIGS. 7 and 8, such an HUD includes a projector 501 for emitting projection light beams L1, L2, and L3, a projection lens 502 for causing images of the projection light beams L1, L2, and L3 to be formed on a transmission type screen 503, the transmission type screen 503 having a back surface that receives the projection light beams L1, L2, and L3 from the projection lens 502 and forms real images thereof and a front surface that diffuses and emits image light beams forming the real images, and a plane mirror 504 and a concave mirror 505 for directing image light beams M1, M2, and M3 diffused on the transmission type screen 503 to a windshield (projection reflection surface) 510 of a vehicle. P1, P2, and P3 shown in FIGS. 7 and 8 indicate optical paths of the image light beams contributing to a viewer visually recognizing a virtual image.
Projection light L directed from the projection lens 502 to the transmission type screen 503 is conically broadened toward the transmission type screen 503. The projection light beam L1 incident on the transmission type screen 503 is diffused by the transmission type screen 503 and is diffused as the image light beam M1 having, as an optical axis, a light distribution axis M1a having the same direction as that of the projection light beam L1. Similarly, the projection light beams L2 and L3 incident on the transmission type screen 503 are also diffused by the transmission type screen 503 and are diffused as the image light beams M2 and M3 having, as optical axes, light distribution axes M2a and M3a having the same directions as those of the projection light beams L2 and L3.